Summary:
This fixes a compilation failure on Linux when the system libc is not
glibc. jemalloc's configure script incorrectly assumes that glibc is
always used on Linux systems, producing glibc-style signatures; when
the system libc is e.g. musl, the following error is observed:

```
  [  0%] Building CXX object CMakeFiles/rocksdb.dir/db/db_impl.cc.o
  In file included from /go/src/github.com/cockroachdb/cockroach/c-deps/rocksdb.src/table/block.h:19:0,
                   from /go/src/github.com/cockroachdb/cockroach/c-deps/rocksdb.src/db/db_impl.cc:77:
  /x-tools/x86_64-unknown-linux-musl/x86_64-unknown-linux-musl/sysroot/usr/include/malloc.h:19:8: error: declaration of 'size_t malloc_usable_size(void*)' has a different exception specifier
   size_t malloc_usable_size(void *);
          ^~~~~~~~~~~~~~~~~~
  In file included from /go/src/github.com/cockroachdb/cockroach/c-deps/rocksdb.src/db/db_impl.cc:20:0:
  /go/native/x86_64-unknown-linux-musl/jemalloc/include/jemalloc/jemalloc.h:78:33: note: from previous declaration 'size_t malloc_usable_size(void*) throw ()'
   #  define je_malloc_usable_size malloc_usable_size
                                   ^
  /go/native/x86_64-unknown-linux-musl/jemalloc/include/jemalloc/jemalloc.h:239:41: note: in expansion of macro 'je_malloc_usable_size'
   JEMALLOC_EXPORT size_t JEMALLOC_NOTHROW je_malloc_usable_size(
                                           ^~~~~~~~~~~~~~~~~~~~~
  CMakeFiles/rocksdb.dir/build.make:350: recipe for target 'CMakeFiles/rocksdb.dir/db/db_impl.cc.o' failed
```

This works around the issue by rearranging the sources such that
jemalloc's headers are never in the same scope as the system's malloc
header. The jemalloc issue has been reported as well, see:
https://github.com/jemalloc/jemalloc/issues/778.

cc tschottdorf
Closes https://github.com/facebook/rocksdb/pull/2188

Differential Revision: D5163048

Pulled By: siying

fbshipit-source-id: c553125458892def175c1be5682b0330d80b2a0d